Wait for CKEditor to load in specs

Some specs involving CKEditor were failing sometimes in the Rails 5.1
branch. The reason why these specs pass with Rails 5.0 but fail with
Rails 5.1 are unknown. On my machine the tests pass when precompiling
the assets, which makes me think it's related to the way Rails handles
them, but it might have nothing to do with it.

The only (apparently) 100% reliable solution I've found is to wait for
CKEditor to load before trying to fill it in. After running the tests on
my machine hundreds of time, I didn't get a single failure.

spec/support/common_actions/verifications.rb

@@ -47,6 +47,10 @@ module Verifications
   def fill_in_ckeditor(text, params = {})
     locator = find("label", text: text)[:for]
 
+    until page.execute_script("return CKEDITOR.instances.#{locator}.status === 'ready';") do
+      sleep 0.01
+    end
+
     # Fill the editor content
     page.execute_script <<-SCRIPT
         var ckeditor = CKEDITOR.instances.#{locator}

spec/system/ckeditor_spec.rb

@@ -19,6 +19,7 @@ describe "CKEditor" do
     login_as(create(:administrator).user)
 
     visit new_admin_site_customization_page_path
+    fill_in_ckeditor "Content", with: "Filling in to make sure CKEditor is loaded"
     find(".cke_button__image").click
 
     expect(page).to have_css(".cke_dialog")
@@ -41,8 +42,9 @@ describe "CKEditor" do
 
   scenario "cannot upload attachments through link tab", :js do
     login_as(create(:administrator).user)
-    visit new_admin_site_customization_page_path
 
+    visit new_admin_site_customization_page_path
+    fill_in_ckeditor "Content", with: "Filling in to make sure CKEditor is loaded"
     find(".cke_button__link").click
 
     expect(page).to have_css(".cke_dialog")